1. Technical Field
The present invention relates to rechargeable battery packs having one or more battery cells disposed within a case or housing. This battery pack may be electrically coupled to a battery charger in order to charge the battery cells. Thereafter, the charged battery pack may be connected to a power tool or another battery-powered appliance in order to supply current to the tool or device.
2. Description of the Related Art
Generally speaking, known rechargeable battery packs are installed in a battery charger in order to re-charge the batteries. A plurality of individual batteries or battery cells may be connected in parallel and/or in series in order to provide the desired battery voltage and output current. During recharging, the battery cells typically generate heat, thereby increasing the temperature of the battery cells. Various arrangements for cooling the battery cells during the recharging operation have been proposed.
Nickel metal hydride batteries provide increased or greater battery capacity (energy density) as compared to other known battery technologies, such as nickel-cadmium batteries, thereby making nickel metal hydride batteries particularly suitable for driving power tools. In addition, nickel metal hydride batteries do not include cadmium, thereby providing a more environmentally friendly power storage device. However, thus far, the use of nickel metal hydride batteries has been limited in the power tool field, because nickel metal hydride batteries are known for generating a relatively large amount of heat when known charging techniques are utilized to re-charge the batteries, especially if a relatively quick charge is performed. If the temperature of the nickel metal hydride battery cells is allowed to become higher than a certain threshold temperature (typically, between about 50°-60° C. for current nickel metal hydride battery technologies), the life of the battery cell may be significantly shortened due to internal damage caused by the relatively high temperature. The nickel metal hydride batteries, of course, also could be charged relatively slowly in order to minimize the likelihood of excessive temperature increases. However, slow charging will naturally reduce the desirability of utilizing nickel metal hydride batteries, because the power tool operator must wait a comparatively longer time to recharge the battery pack for further use.
Thus, there is a long-felt need in the power tool field, as well as other fields that utilize rechargeable batteries, to develop battery pack designs and battery charging technologies that will enable nickel metal hydride batteries, or other battery types that become hot during recharging, to be quickly charged without overheating and thus damaging the battery cells.
Furthermore, battery-driven power tools generally must be operated using relatively large currents in order to operate with the same efficiency and effectiveness as power tools driven by a commercial AC power source. Thus, if a short circuit develops within the battery pack, serious problems could result due to the relatively high currents that can be supplied by nickel metal hydride batteries. Therefore, the battery cells are preferably isolated or shielded from outside moisture and foreign substances in order to prevent or reduce the possibility of short circuits within the battery pack. Moreover, it is preferable to uniformly cool the battery cells during the recharging operation so that all the battery cells are maintained at substantially the same temperature. In this case, it is possible to avoid the possibility that one or more battery cells will reach a temperature that will cause permanent damage to the battery cell, and thereby make the battery pack inoperative for its intended purpose.
European Patent Publication No. 0 940 864 describes a battery pack structure for nickel metal hydride battery cells. However, this known design focuses primarily on cooling the battery cells and does not teach any techniques for protecting the battery cells from moisture and foreign substances. In fact, the battery cells of European Patent Publication No. 0 940 864 are cooled by directly contacting the battery cells with forced air supplied from the battery charger and/or the power tool. Thus, moisture or foreign substances can easily contact the battery terminals and cause degradation, which may lead to short circuits. Further, the battery packs of European Patent Publication No. 0 940 864 rely upon metal heat sinks in order to uniformly cool the battery cells within the battery pack. However, a metal heat sink will naturally increase the overall weight of the battery pack, as well as the cost of manufacturing the battery packs.
In European Patent Publication No. 0 994 523, the present Applicant proposed a battery pack design in which a plurality of battery cells is disposed within a dual-wall housing. An inner case optionally may be formed, either entirely or partially, from a thermally conductive material, such as aluminum. Further, the inner case may directly contact the battery cells in order to uniformly cool the battery cells. In addition, the inner case may substantially surround or enclose the battery cells in order to protect the battery cells from outside moisture and foreign substances. Moreover, the inner case may be housed or disposed within an outer case and a cooling-air passage may be defined between the inner and outer cases. Thus, the battery pack design of European Patent Publication No. 0 994 523 enables uniform cooling of the battery cells while preventing moisture and foreign substances from contacting the battery terminals. Further, the double housing serves to protect the power tool operator in the event that a short circuit happens to develop between the battery cells.
Thus, European Patent Publication No. 0 994 523 provides a commercially useful battery pack design, which effectively cools nickel metal hydride batteries during the recharging operation and effectively prevents degradation that could lead to dangerous short circuits.